Method of direct electrodeposition on semiconductors

ABSTRACT

The present disclosure provides a method of electrodeposition of a metal or metal alloy on at least one surface of a semiconductor material. The method of the present invention provides full coverage of an electrodeposited metallic film on the at least one surface of the semiconductor material. The method of the present disclosure includes providing a semiconductor material. A metallic film is applied to at least one surface of the semiconductor material by an electrodeposition process. The electrodeposition process employed uses current waveforms that apply a low current density initially, and after a predetermined period of time, the current density is changed to a high current density.

BACKGROUND

The present invention relates to a method of depositing a metal or metal alloy on a surface of a semiconductor material, and more particularly to a method of electrodepositing a metal or metal alloy on a surface of a doped semiconductor material. The method of the present disclosure can be integrated with any conventional solar cell fabrication process to provide a solar cell or photovoltaic cell including a doped semiconductor material having an electrodeposited metal or metal alloy on a surface thereof.

Electroplating is a plating process that uses electrical current to reduce cations of a desired material from a solution and coat an object, typically a conductive object, with a thin layer of the material, such as a metal. Electroplating is primarily used for depositing a layer of material to bestow a desired property (e.g., good electrical conductivity, abrasion and wear resistance, corrosion protection, lubricity, aesthetic qualities, etc.) to a surface that otherwise lacks that property.

The process used in electroplating is called electrodeposition. It is analogous to a galvanic cell acting in reverse. The part to be plated is typically the cathode of the circuit. In one technique, the anode is made of a metal to be plated on a part. In other techniques, the anode is made of an inert metal, which can not be dissolved in the electrolyte during electrodeposition. Both components are immersed in a solution called an electrolyte containing one or more dissolved metal salts as well as other ions that permit the flow of electricity. A battery or rectifier supplies a direct current to the anode, oxidizing the metal atoms that comprise it and allowing them to dissolve in the solution in the case of a soluble anode. In the case of an inert anode, water is being oxidized to oxygen. At the cathode, the dissolved metal ions in the electrolyte solution are reduced at the interface between the solution and the cathode, such that they “plate out” onto the cathode.

Electrodeposition directly on a semiconducting surface, e.g., a semiconductor substrate, is more complicated than on a conductive surface, i.e., a metal. The presence of a bandgap on a semiconductor material makes nucleation more difficult and more sensitive to the states of the surface. For example, when depositing on an n-doped silicon, i.e., n—Si, surface, the dopant density will affect the nucleation rate. Any non-uniformity of the dopant profile will lead to non-uniform deposition of metals and metal alloys. In another example, and when the semiconductor surface has surface states due to contamination or initial metal nuclei, the deposition will occur preferentially at the sites with viable surface sites. See, for example, G. Oskam et al., Phy. Rev. Lett. 76 (1996), pg. 1521. This also leads to non-plated areas.

In light induced plating for solar or photovoltaic cell metallization, surface contamination and dopant non-uniformity are common phenomena. As such, it is quite difficult to achieve full metal coverage on an n-emitter semiconductor surface of a solar or photovoltaic cell. By using a constant current or potential control electrodeposition technique, large portions of an n-emitter semiconductor surface may not be able to be electrodeposited with metal films when the n-emitter semiconductor surface is not uniform.

SUMMARY

The present disclosure provides a method of electrodeposition of a metal or metal alloy on at least one surface of a semiconductor material. The method of the present invention provides full coverage of an electrodeposited metallic film on the at least one surface of the semiconductor material. The method of the present invention can be used to fully cover an n-emitter surface of a solar or photovoltaic cell. As such, an improved and lower cost technique for metallization is provided by the present disclosure which can be used in the photovoltaic industry in place of current screen printing processes.

The method of the present disclosure includes providing a semiconductor material. In some embodiments, the semiconductor material may be doped. In other embodiments, the semiconductor material may be undoped. A metallic film is applied to at least one surface of the semiconductor material by an electrodeposition process. The electrodeposition process employed uses current waveforms that apply a first current density for a first period of time, followed by a second current density for a second period of time, wherein the first current density is lower than the second current density. Specifically, the electrodeposition process of the present disclosure uses current waveforms that apply a low current density initially, and after a predetermined period of time, the current density is ramp-up to a high current density. The applicants of the present disclosure have determined that the use of the aforementioned current waveform (e.g., low current density to high current density) overcomes the non-uniformity problem that exists during prior art electrodeposition processes.

In one embodiment, the current waveform employed in the present disclosure can be a continuous ramp from a low current density to a high current density. In another embodiment, the current waveform can be a sequence of constant current plateaus, starting from a low current density to a high current density. The term “low current density” as used throughout the present disclosure denotes a current density from 5 mAmps/cm² to 40 mAmps/cm². The term “high current density” as used throughout the present disclosure denotes a current density of greater than 40 mAmps/cm², with a current density from greater than 40 mAmps/cm² to 200 mAmps/cm² being a typical range for the high current density.

The use of the aforementioned current waveform (i.e., from a low current density to a high current density) to overcome the non-uniformity problem is quite contradictory to common knowledge in the electrodeposition field since most prior art electrodeposition processes use the opposite current waveform sequence, i.e., from a high current density to a low current density. In such prior art electrodeposition processes, the higher current density is initially used to initiate high density nuclei, and then a lower current density is used to grow the film.

In some embodiments of the invention, light illumination can be used to increase metal nucleation and growth during the electrodeposition process. In particular, light illumination can be used in embodiments in which solar or photovoltaic cells are to be fabricated to generate free electrons that can be used during the electrodeposition process.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a pictorial representation (through a cross sectional view) illustrating an initial structure including at least a semiconductor material that can be employed in one embodiment of the invention.

FIG. 2 is a pictorial representation (through a cross sectional view) depicting the initial structure of FIG. 1 after forming an optional patterned antireflective coating (ARC) on one surface of the semiconductor material.

FIG. 3A is a pictorial representation (through a cross sectional view) depicting the initial structure of FIG. 1 after forming a metallic film on one surface of the semiconductor material utilizing an electrodeposition process in accordance with the present invention.

FIG. 3B is a pictorial representation (through a cross sectional view) depicting the structure of FIG. 2 after forming a metallic film on one surface of the semiconductor material utilizing an electrodeposition process in accordance with the present invention.

FIG. 4 is a SEM showing the Ni plating on a Si solar cell n-emitter grid surface using a high current density to plate the Ni as in accordance with a prior art method.

FIG. 5 is a SEM showing the Ni plating on a Si solar cell n-emitter grid surface using a waveform current density from low to high to plate the Ni as in accordance with the present disclosure.

FIG. 6 is a SEM showing the Ni plating on a Si solar cell n-emitter grid surface using a waveform current density from high to low to plate the Ni as in accordance with another prior art method.

DETAILED DESCRIPTION

The present disclosure, which provides a method of electrodeposition of a metal or metal alloy directly on a surface of a semiconductor material, will now be described in greater detail by referring to the following discussion and drawings that accompany the present application. It is observed that the drawings of the present application are provided for illustrative proposes and, as such, the drawings are not drawn to scale.

In the following description, numerous specific details are set forth, such as particular structures, components, materials, dimensions, processing steps and techniques, in order to provide an understanding of some aspects of the present invention. However, it will be appreciated by one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known structures or processing steps have not been described in detail in order to avoid obscuring the invention.

It will be understood that when an element as a layer, region or substrate is referred to as being “on” or “over” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “beneath” or “under” another element, it can be directly beneath or under the other element, or intervening elements may be present. In contrast, when an element is referred to as being “directly beneath” or “directly under” another element, there are no intervening elements present.

As stated above, the present disclosure provides a method of electrodeposition of a metal or metal alloy on at least one surface of a semiconductor material in which full coverage of an electrodeposited metallic film is achieved. The method includes providing a semiconductor material. A metallic film is then applied to at least one surface of the semiconductor material by an electrodeposition process, wherein current waveforms are employed that apply a low current density initially, and after a predetermined period of time, the current density is increased to a high current density.

Referring now to FIG. 1, there is illustrated an initial structure 8 including a semiconductor material 10 that can be employed in one embodiment of the invention. The semiconductor material 10 has at least one surface 12 in which a metallic film will be subsequently formed thereon using the electrodeposition method of the present disclosure. The semiconductor material 10 employed includes, but not limited to, Si, Ge, SiGe, SiC, SiGeC, GaAs, GaN, InAs, InP and all other III/V or II/VI compound semiconductors. Semiconductor material 10 may also comprise an organic semiconductor or a layered semiconductor such as Si/SiGe, a silicon-on-insulator (SOI), a SiGe-on-insulator (SGOI) or a germanium-on-insulator (GOI). In one embodiment of the present invention, the semiconductor material 10 is comprised of Si. In one embodiment, the semiconductor material 10 is comprised of a single crystalline semiconductor material. In another embodiment, the semiconductor material 10 is comprised of a multicrystalline semiconductor material. In another embodiment of the present application, the semiconductor material 10 may comprise a substrate in which at least one device, including a solar or photovoltaic cell, can be formed there upon.

The semiconductor material 10 may be doped with the same or different conductivity type, e.g., n-type and/or p-type dopant, undoped or contain doped and undoped regions therein. In the particular embodiment illustrated in FIG. 1, the semiconductor material 10 includes a p-type semiconductor portion 10A that includes a p-type dopant, and an overlying n-type semiconductor portion 10B that includes an n-type dopant. The term “n-type dopant” is used throughout the present disclosure to denote an atom from Group VA of the Periodic Table of Elements including, for example, P, As and/or Sb. The term “p-type dopant” is used throughout the present disclosure to denote an atom from Group IIIA of the Periodic Table of Elements including, for example, B, Al, Ga and/or In.

The concentration of dopant within the semiconductor material may vary depending on the ultimate end use of the semiconductor material and the type of dopant atom being employed. In the particular embodiment shown in FIG. 1, the p-type semiconductor portion 10A of the semiconductor material 10 typically has a p-type dopant concentration from 1.0E12 atoms/cm³ to 1E22 atoms/cm³, with a p-type dopant concentration from 1.0E16 atoms/cm³ to 1.0E20 atoms/cm³ being more typical. The n-type semiconductor portion 10B of the semiconductor material 10 typically has an n-type dopant concentration from 1.0E11 atoms/cm³ to 1.0E22 atoms/cm³, with an n-type dopant concentration from 1.0E13 atoms/cm³ to 1.0E20 atoms/cm³ being more typical.

When the semiconductor material 10 is doped, the n-type and/or p-type dopant can be introduced into the semiconductor material using techniques well known to those skilled. For example, the n-type and/or p-type dopant can be introduced into the semiconductor material by ion implantation, gas phase doping, liquid spray/mist doping, and/or out-diffusion of a dopant atom from an overlying sacrificial dopant material layer that can be formed on the substrate, and removed after the out-diffusion process. In some embodiments of the invention, the dopant(s) can be introduced into the semiconductor material 10 during the formation thereof. For example, an in-situ epitaxial growth process can be used to form a doped semiconductor material 10.

The at least one surface 12 of the semiconductor material 10 may be non-textured or textured. A textured (i.e., specially roughened) surface is typically used in cases in which the semiconductor material 10 is used in solar cell applications to increase the efficiency of light absorption. The textured surface decreases the fraction of incident light lost to reflection relative to the fraction of incident light transmitted into the cell since photons incident on the side of an angled feature will be reflected onto the sides of adjacent angled features and thus have another chance to be absorbed. Moreover, the textured surface increases internal absorption, since light incident on an angled silicon surface will typically be deflected to propagate through the substrate at an oblique angle, thereby increasing the length of the path taken to reach the substrates back surface, as well as making it more likely that photons reflected from the substrate back surface will impinge on the front surface at angles compatible with total internal reflection and light trapping. The texturing of the at least one surface 12 of the semiconductor material 10 can be performed utilizing conventional techniques well known in the art. In one embodiment, a KOH based solution can be used to texture the at least one surface 12 of the semiconductor material 10. In another embodiment, texturing can be achieved by utilizing a HNO₃/HF based solution on the at least one surface 12 of the semiconductor material 10. In another embodiment, texturing can be achieved by utilizing a combination of reactive ion etching (RIE) and a mask comprising closely packed self-assembled polymer spheres.

In embodiments in which the semiconductor material 10 is employed as a substrate for use in a solar cell, a metallic paste (not shown) is applied to a surface of the semiconductor material that is opposite the at least one surface in which the metallic film will be subsequently electrodeposited thereon. The metallic paste, which includes any conductive paste such as Al, Ag, or AlAg paste, is formed utilizing conventional techniques that are well known to those skilled in the art of solar cell fabrication. After applying the metallic paste, the metallic paste is heated to a sufficiently high temperature which causes the metallic paste to flow and form a metallic layer on the applied surface of the semiconductor material. In one embodiment in which a Si material is employed as the semiconductor material 10 and where an Al paste is employed, the Al paste is heated to a temperature from 700° C. to 900° C. which causes the Al paste to flow and form a metallic Al and AlSi layer. The metallic Al and AlSi layer that is formed from the metallic paste serves as a conductive back surface field or backside electrical contact of a solar cell.

Referring now to FIG. 2, there is illustrated the structure that is formed after forming an optional patterned antireflective coating (ARC) 14 on the at least one surface 12 of the semiconductor material 10. As shown, the optional patterned ARC 14 has at least one opening therein that exposes portions of the semiconductor material 10. The patterned ARC 14 that can be employed in the present invention includes any conventional ARC material including for example inorganic ARCs and organics ARCs. The patterned ARC 14 can be formed utilizing techniques well known to those skilled in the art. For example, an ARC composition can be applied to the at least one surface 12 of the semiconductor material 10 utilizing a conventional deposition process including, for example, spin-on coating, dip coating, evaporation, chemical solution deposition, chemical vapor deposition (CVD), and plasma enhanced chemical vapor deposition (PECVD). After application of the ARC composition, particularly those from a liquid phase, a post deposition baking step is usually employed to remove unwanted components, such as solvent, and to effect crosslinking. The post deposition baking step of the ARC composition is typically, but not necessarily always, performed at a temperature from 80° C. to 300° C., with a baking temperature from 120° C. to 200° C. being more typical.

In some embodiments, the as-deposited ARC composition may be subjected to a post deposition treatment to improve the properties of the entire layer or the surface of the ARC 14. This post deposition treatment can be selected from heat treatment, irradiation of electromagnetic wave (such as ultra-violet light), particle beam (such as an electron beam, or an ion beam), plasma treatment, chemical treatment through a gas phase or a liquid phase (such as application of a monolayer of surface modifier) or any combination thereof. This post-deposition treatment can be blanket or pattern-wise.

The applied ARC composition can be patterned utilizing lithography and etching. The lithographic process includes applying a photoresist (not shown) to an upper surface of the as-deposited ARC composition, exposing the photoresist to a desired pattern of radiation and developing the exposed photoresist utilizing a conventional resist developer. A patterned photoresist is thus provided. The pattern in the photoresist is transferred to the as-deposited ARC composition utilizing an etching process such as, for example, dry etching or chemical wet etch. After transferring the pattern from the patterned photoresist to the underlying as-deposited ARC composition, the patterned photoresist is typically removed from the structure utilizing a conventional resist stripping process such as, for example, ashing. In another embodiment, the ARC layer can be patterned utilizing ink jet printing or laser ablation.

It is observed that the patterned ARC 14 is typically employed in embodiments in which the semiconductor material 10 is to be used as a substrate of a solar or photovoltaic cell.

Referring now to FIGS. 3A and 3B, there are illustrated the structures that are formed after a metallic film 16 is formed on exposed surfaces of the semiconductor material 10 utilizing the electrodeposition method of the present application. The metallic film 16 that is formed may comprise any metal or metal alloy. In one embodiment of the present application, the metallic film 16 is comprised of Ni, Co, Cu, Zn, Pt, Ag, Pd, Sn, Fe, In or alloys thereof. In another embodiment, the metallic film 16 is comprised of Ni, Co, Cu, Zn, Pt, Fe or alloys thereof. In a further embodiment of the present invention, the metallic film 16 is comprised of Ni or a Ni alloy.

In some embodiments, and prior to the electrodeposition of metallic film 16, the exposed surface(s) of the semiconductor material 10 is cleaned using a conventional cleaning process that is well known to those skilled in the art which is capable of removing surface oxides and other contaminants from the exposed surface(s) of the semiconductor material. For example, a diluted HF solution can be used to clean the exposed surface(s) of the semiconductor material 10.

The electrodeposition method of the present application used in forming metallic film 16 includes the use of any conventional electrodeposition or electroplating apparatus that is well known to those skilled in the art. A soluble or insoluble anode may be used.

The electrodeposition method of the present application also includes the use of any conventional electroplating bath (or composition). The electroplating bath includes one or more sources of metal ions to plate metals. The one or more sources of metal ions provide metal ions which include, but are not limited to Ni, Co, Cu, Zn, Pt, Ag, Pd, Sn, Fe and In. Alloys that can be electrodeposited (or plated) include, but are not limited to, binary and ternary alloys of the foregoing metals. Typically, metals chosen from Ni, Co, Cu, Zn, Pt and Fe are plated from the electroplating bath. More typically, Ni or a Ni alloy is plated from the electroplating bath.

The one or more sources of metal ions that can be present in the electroplating bath include metal salts. The metal salts that can be used include, but are not limited to, metal halides, metal nitrates, metal sulfates, metal sulfamates, metal alkane sulfonates, metal alkanol sulfonate, metal cyanides, metal acetates or metal citrates.

Some of the various types of metal salts that can be employed in the present invention are now described in greater detail. Copper (Cu) salts which may be used in the electroplating bath include, but are not limited to, one or more of copper halides, copper sulfates, copper phosphates, copper acetates, and copper citrate. Typically, copper sulfate, copper phosphates, or copper citrates, or mixtures thereof are used in the electroplating bath.

Tin (Sn) salts which may be used in the electroplating bath include, but are not limited to, one or more of tin sulfates, tin halides, tin alkane sulfonates such as tin methane sulfonate, tin ethane sulfonate, and tin propane sulfonate, tin aryl sulfonate such as tin phenyl sulfonate and tin toluene sulfonate, and tin alkanol sulfonate. Typically, tin sulfate or tin alkane sulfonate is used in the electroplating bath.

Gold (Au) salts which may be used in the electroplating bath include, but are not limited to, one or more of gold trichloride, gold tribromide, gold cyanide, potassium gold chloride, potassium gold cyanide, sodium gold chloride and sodium gold cyanide.

Silver (Ag) salts which may be used in the electroplating bath include, but are not limited to, one or more of silver nitrate, silver chloride, silver acetate and silver bromate. Typically, silver nitrate is used in the electroplating bath.

Nickel (Ni) salts which may be used in the electroplating bath include, but are not limited to, one or more of nickel chloride, nickel sulfamate, nickel acetate, nickel ammonium sulfate, and nickel sulfate.

Palladium (Pd) salts which may be used in the electroplating bath include, but are not limited to, one or more of palladium chloride, palladium nitrate, palladium potassium chloride and palladium potassium chloride.

Platinum (Pt) salts which may be use include, but are not limited to, one or more of platinum tetrachloride, platinum sulfate and sodium chloroplatinate.

Indium (In) salts which may be used include, but are not limited to, one or more of indium salts of alkane sulfonic acids and aromatic sulfonic acids, such as methanesulfonic acid, ethanesulfonic acid, butane sulfonic acid, benzenesulfonic acid and toluenesulfonic acid, salts of sulfamic acid, sulfate salts, chloride and bromide salts of indium, nitrate salts, hydroxide salts, indium oxides, fluoroborate salts, indium salts of carboxylic acids, such as citric acid, acetoacetic acid, glyoxylic acid, pyruvic acid, glycolic acid, malonic acid, hydroxamic acid, iminodiacetic acid, salicylic acid, glyceric acid, succinic acid, malic acid, tartaric acid, hydroxybutyric acid, indium salts of amino acids, such as arginine, aspartic acid, asparagine, glutamic acid, glycine, glutamine, leucine, lysine, threonine, isoleucine, and valine.

Sources of cobalt (Co) ions include, but are not limited to, one or more of cobalt ammonium sulfate, cobalt acetate, cobalt sulfate and cobalt chloride. Sources of zinc (Zn) ions include, but are not limited to, one or more of zinc bromate, zinc chloride, zinc nitrate and zinc sulfate. Source of iron (Fe) include, but are not limited to, one or more of ferric or ferrous chloride, iron nitrate, iron sulfate, iron acetate, and iron sulfate.

In general, the metal salts are included in the electroplating bath such that metal ions range in concentrations from 0.01 g/L to 200 g/L, or such as from 0.5 g/L to 150 g/L, or such as from 1 g/L to 100 g/L, or such as from 5 g/L to 50 g/L. Typically, metal salts are included in amounts such that metal ion concentrations range from 0.01 to 100 g/L, more typically from 0.1 g/L to 60 g/L.

The electroplating bath that can be used may include one or more conventional diluents. Typically, the electroplating bath is aqueous; however, conventional organic diluents may be used if desired. Optional conventional electroplating bath additives also may be included. Such additives include, but are not limited to, one or more of brighteners, suppressors, surfactants, inorganic acids, organic acids, brightener breakdown inhibition compounds, alkali metal salts, and pH adjusting compounds. Additional additives may be included in the metal plating baths to tailor the performance of the metal plating for a particular substrate. Such additional additives may include, but are not limited to, levelers and compounds which affect throwing power.

Brighteners that can be employed include, but are not limited to, one or more of 3-mercapto-propylsulfonic acid sodium salt, 2-mercapto-ethanesulfonic acid sodium salt, bissulfopropyl disulfide (BSDS), N,N-dimethyldithiocarbamic acid (3-sulfopropyl) ester sodium salt (DPS), (O-ethyldithiocarbonato)-S-(3-sulfopropyl)-ester potassium salt (OPX), 3-[(amino-iminomethyl)-thio]-1-propanesulfonic acid (UPS), 3-(2-benzthiazolylthio)-1-propanesulfonic acid sodium salt (ZPS), the thiol of bissulfopropyl disulfide (MPS), sulfur compounds such as 3-(benzthiazoyl-2-thio)-propylsulfonic acid sodium salt, 3-mercaptopropane-1-sulfonic acid sodium salt, ethylenedithiodipropylsulfonic acid sodium salt, bis-(p-sulfophenyl)-disulfide disodium salt, bis-(ω-sulfobutyl)-disulfide disodium salt, bis-(ω-sulfohydroxypropyl)-disulfide disodium salt, bis-(ω-sulfopropyl)-disulfide disodium salt, bis-(ω-sulfopropyl)-sulfide disodium salt, methyl-(ω-sulfopropyl)-disulfide sodium salt, methyl-(ω-sulfopropyl)-trisulfide disodium salt, O-ethyl-dithiocarbonic acid-S-(ω-sulfopropyl)-ester, potassium salt thioglycolic acid, thiophosphoric acid-O-ethyl-bis-(ω-sulfpropyl)-ester disodium salt, and thiophosphoric acid-tris(ω-sulfopropyl)-ester trisodium salt. Brighteners may be added to the electroplating bath in conventional amounts, In general, brighteners are added in amounts of 1 ppb to 1 g/L, or such as from 10 ppb to 500 ppm.

Suppressors include, but are not limited to, one or more of oxygen containing high molecular weight compounds such as carboxymethylcellulose, nonylphenolpolyglycol ether, octandiolbis-(polyalkylene glycolether), octanolpolyalkylene glycolether, oleic acidpolyglycol ester, polyethylenepropylene glycol, polyethylene glycol, polyethylene glycoldimethylether, polyoxypropylene glycol, polypropylene glycol, polyvinylalcohol, stearic acidpolyglycol ester, and stearyl alcoholpolyglycol ether. Typically poly(alkoxylated)glycols are used. Such suppressors may be included in the electroplating bath in conventional amounts, such as from 0.01 g/L to 10 g/L, or such as from 0.5 g/l to 5 g/L.

One or more conventional surfactants may be used. Typically, surfactants include, but are not limited to, nonionic surfactants such as alkyl phenoxy polyethoxyethanols. Other suitable surfactants containing multiple oxyethylene groups also may be used. Such surfactants include compounds of polyoxyethylene polymers having from as many as 20 to 7500 repeating units. Such compounds also may perform as suppressors. Also included in the class of polymers are both block and random copolymers of polyoxyethylene (EO) and polyoxypropylene (PO). Surfactants may be added in conventional amounts, as from 0.5 g/L to 20 g/L, or such as from 5 g/L to 10 g/L.

Conventional levelers include, but are not limited to, one or more of alkylated polyalkyleneimines and organic sulfo sulfonates. Examples of such compounds include 1-(2-hydroxyethyl)-2-imidazolidinethione (HIT), 4-mercaptopyridine, 2-mercaptothiazoline, ethylene thiourea, thiourea, 1-(2-hydroxyethyl)-2-imidazolidinethione (HIT) and alkylated polyalkyleneimines. Such levelers are included in conventional amounts. Typically, such levelers are included in amounts of 1 ppb to 1 g/L, or such as from 10 ppb to 500 ppm.

One or more inorganic and organic acids can be also included in the electroplating bath to increase the solution conductivity of the matrix and also to adjust the pH of the plating composition. Inorganic acids include, but are not limited to, sulfuric acid, hydrochloric acid, nitric acid and phosphoric acid. Organic acids include, but are not limited to, alkane sulfonic acids, such a methane sulfonic acid. Acids are included in the electroplating bath in conventional amounts.

Alkali metal salts which may be included in the electroplating bath include, but are not limited to, sodium and potassium salts of halogens, such as chloride, fluoride and bromide. Typically chloride is used. Such alkali metal salts are used in conventional amounts.

In addition to the above, the electroplating bath may also include hardeners, malleability, ductility and deposition modifiers, suppressants and the like.

The measured pH of the electroplating bath may range from −1 to 14, or such as from −1 to 8. Typically, the pH of the electroplating bath ranges from −1 to 5, more typically, from −1 to 3. Conventional buffering compounds may be included to control the pH of the electroplating bath.

The electroplating baths are typically maintained in a temperature range of from 20° C. to 110° C., with a temperature from 20° C. to 50° C. being more typical. Plating temperatures may vary depending on the metal to be plated.

The electrodeposition process employed in forming the metallic film 16 uses current waveforms that apply a low current density initially, and after a predetermined period of time, the current density is increased to a high current density. The specific waveforms that are employed can be continuously applied or pulsed waveforms can be employed in the present invention. It has been determined by the applicants of the present disclosure that the use of the aforementioned current waveform (e.g., low current density to high current density) overcomes the non-uniformity problem that exists during prior art electrodeposition processes.

The low current density that is initially used to plate the metal or metal alloy from the plating bath is typically within a range from 1 mAmps/cm² to 40 mAmps/cm², with a current density from 5 mAmps/cm² to 20 mAmps/cm² being more typical. Plating within the low current density regime is typically performed for a time period from 5 seconds to 120 seconds, with a time period from 10 seconds to 60 seconds being more typical. After this initial period of time in which plating occurs using the low current density mentioned above, the current density is increased to a high current density regime. The high current density regime typically employs a current density of greater than 40 mAmps/cm², with a current density from greater than 40 mAmps/cm² to 200 mAmps/cm² being more typical. Plating within the high current density regime is typically performed for a time period from 1 second to 1 hour, with a time period from 5 seconds to 300 seconds being more typical.

The increase from the low current density regime to the high current density regime may include a continuous ramp or it may include various ramp and soak cycles including a sequence of constant current plateaus. When a continuous ramp is employed, the rate of increase can be from 1 mAmp/cm²/sec to 100 mAmp/cm²/sec. The same ramp rate can be used in the various ramp and soak cycles and the soak at a desired current density may vary and is not critical to the practice of the present invention.

The use of the aforementioned current waveform (i.e., from a low current density to a high current density) overcomes the non-uniformity problem that is typically observed using other waveforms.

The thickness of the metallic film 16 that is electrodeposited using the above described conditions may vary depending on the type of metal being electrodeposition, the type of electroplating bath employed as well as the duration of the electrodeposition process itself. Typically, the metallic film 16 that is formed from the electrodeposition described in this disclosure is from 50 Å to 50000 Å, with a thickness from 500 Å to 5000 Å being more typical. Moreover, the electrodeposition method described above provides complete coverage of the electrodeposited metallic film 16 on the exposed surface of the semiconductor material. By “complete coverage”, it is meant that no exposed substrate areas are present.

In some embodiments of the invention, light illumination can be used to increase metal nucleation and growth during the electrodeposition process. In particular, light illumination can be used in embodiments in which solar or photovoltaic cells are to be fabricated to generate free electrons that can be used during the electrodeposition process. When light illumination is employed during the electrodeposition process, any conventional light source can be used. The intensity of the light employed may vary and is typically greater than 5000 Lux, with an intensity of light from 10000 Lux to 50000 Lux being more typical. The combination of the aforementioned waveform and light illumination enables one to provide complete coverage of a metallic film on the surface of a semiconductor substrate used in solar cell applications.

The following examples are provided to illustrate some advantages that can be obtained using the electrodeposition method described above.

EXAMPLE

In this example, Ni was electrodeposited on a Si solar cell n-emitter surface similar to the one depicted in FIG. 2 of the present application using different waveforms. In each of the experiments a plating bath consisting of nickel sulfamate and boric acid was employed. The temperature of the plating bath for each experiment was 21° C. Ni was plated from the plating bath at a plating temperature of 21° C.

In a first experiment (i.e., comparative experiment 1), a high current density of 80 mAmps/cm² was employed to plate the Ni. Plating was performed at the aforementioned high current density for a time period of 60 seconds. Using such plating conditions, a large area of the Si solar cell n-emitter surface was not covered as is shown in FIG. 4.

In a second experiment (i.e., experiment 1), a waveform current density (from a low current density to a high current density) in accordance with the present application was employed. In particular, an initial current density of 20 mAmps/cm² was employed to initiate the plating of Ni. The initial current density was held constant for a time period of about 40 seconds. After the initial plating at the low current density, the current density was abruptly ramp-up to a current density of 80 mAmps/cm². Plating was performed at the aforementioned high current density for a time period of 10 seconds. Using such plating conditions, a complete coverage of Ni on the entire exposed surfaces of the Si solar cell n-emitter surface was observed as is shown in FIG. 5.

In a third experiment (i.e., comparative experiment 2), a waveform current density opposite of that of the present application was employed. That is, a waveform current density from high to low was employed in comparative experiment 2. In particular, an initial current density of 80 mAmps/cm² was employed to initiate the plating of Ni. The initial current density was held constant for a time period of about 10 seconds. After the initial plating at the high current density, the current density was decreased to a current density of 20 mAmps/cm² abruptly. Plating was performed at the aforementioned low current density for a time period of 40 seconds. Using such plating conditions, an uneven nucleation and growth of Ni such as shown in FIG. 6 was observed.

While the present invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims. 

1. A method of forming a metallic film on a surface of a semiconductor material comprising: providing a semiconductor material having at least one surface; and electrodepositing a metallic film on the at least one surface of the semiconductor material, wherein said electrodepositing includes electroplating a metal or metal alloy from an electroplating bath in which a first current density is employed for a first period of time, and after said first period of time, a second current density is applied for a second period of time, wherein said first current density is lower than the second current density.
 2. The method of claim 1 wherein said providing the semiconductor material includes selecting from one of Si, Ge, SiGe, SiC, SiGeC, GaAs, GaN, InAs, InP and all other III/V and II/VI compound semiconductors.
 3. The method of claim 1 wherein said providing the semiconductor material includes selecting an n-type doped semiconductor material or a p-type doped semiconductor material.
 4. The method of claim 1 wherein said providing the semiconductor material includes selecting a semiconductor material including a p-type semiconductor portion that is doped with a p-type dopant, and an overlying n-type semiconductor portion that is doped with an n-type dopant.
 5. The method of claim 1 further comprising forming a patterned antireflective coating on a portion of said at least one surface of said semiconductor material.
 6. The method of claim 1 wherein said electroplating bath includes metal salts of Ni, Co, Cu, Zn, Pt, Ag, Pd, Sn, Fe, In or alloys thereof.
 7. The method of claim 6 wherein said electroplating bath includes metal salts of Ni or a Ni alloy.
 8. The method of claim 6 wherein said electroplating bath further includes one or more brighteners, suppressors, surfactants, inorganic acids, organic acids, alkali metal salts, and pH adjusting compounds.
 9. The method of claim 1 wherein said first current density is within a range from 5 mAmps/cm² to 40 mAmps/cm², and said second current density is greater than 40 mAmps/cm².
 10. The method of claim 9 wherein said second current density is from greater than 40 mAmps/cm² to 200 mAmps/cm².
 11. The method of claim 1 wherein said first period of time is from 5 seconds to 120 seconds, and the second period of time is from 1 second to 1 hour.
 12. The method of claim 1 wherein said first current density is continuously ramped up to said second current density.
 13. The method of claim 1 wherein said first current density is ramped up to the second current density utilizing various ramp-up and soak cycles.
 14. The method of claim 1 wherein said electrodepositing is performed in the presence of light having an intensity of greater than 5000 Lux.
 15. A method of forming a metallic film on a surface of a semiconductor material comprising: providing a semiconductor material having at least one surface; and electrodepositing a metallic film on the at least one surface of the semiconductor material, wherein said electrodepositing includes a waveform comprising a low current density of from 5 mAmps/cm² to 40 mAmps/cm² performed for a first period of time, followed by a high current density of greater than 40 mAmps/cm² performed for a second period of time.
 16. The method of claim 15 wherein said providing the semiconductor material includes selecting from one of Si, Ge, SiGe, SiC, SiGeC, GaAs, GaN, InAs, InP and all other III/V and II/VI compound semiconductors.
 17. The method of claim 15 wherein said providing the semiconductor material includes selecting an n-type doped semiconductor material or a p-type doped semiconductor material.
 18. The method of claim 15 wherein said providing the semiconductor material includes selecting a semiconductor material including a p-type semiconductor portion that is doped with a p-type dopant, and an overlying n-type semiconductor portion that is doped with an n-type dopant.
 19. The method of claim 15 further comprising forming a patterned antireflective coating on a portion of said at least one surface of said semiconductor material.
 20. The method of claim 15 wherein said electroplating bath includes metal salts of Ni, Co, Cu, Zn, Pt, Ag, Pd, Sn, Fe, In or alloys thereof.
 21. The method of claim 20 wherein said electroplating bath includes metal salts of Ni or a Ni alloy.
 22. The method of claim 20 wherein said electroplating bath further includes one or more brighteners, suppressors, surfactants, inorganic acids, organic acids, alkali metal salts, and pH adjusting compounds.
 23. The method of claim 15 wherein said second current density is from greater than 40 mAmps/cm² to 200 mAmps/cm².
 24. The method of claim 15 wherein said first period of time is from 5 seconds to 120 seconds, and the second period of time is from 1 second to 1 hour.
 25. The method of claim 15 wherein said first current density is continuously ramped up to said second current density.
 26. The method of claim 15 wherein said first current density is ramped up to the second current density utilizing various ramp-up and soak cycles.
 27. The method of claim 15 wherein said electrodepositing is performed in the presence of light having an intensity of greater than 5000 Lux. 